As a method of enhancing throughput in wireless LAN system, a block ACK scheme is proposed in IEEE802.11e as a technique for integrating a plurality of ACKs into a single frame and enhancing channel efficiency (non-patent document 1). An ACK is a signal returned to a transmission station when a receiving station checks for errors in received data and finds no errors.
Two schemes referred to as “immediate block ACK” and “delayed block ACK” are proposed in relation to the block ACK scheme. In the immediate Block ACK scheme as shown in FIG. 1, when obtaining transmission opportunity (TXOP), the transmission station transmits a plurality of units of QoS data in sequence, and transmits a block ACK Request for requesting transmission of a block ACK to the receiving station as the final sequential transmission. When receiving the block ACK request, the receiving station immediately integrates ACKs for the received plurality of units of data (Data #1 through Data #5 in FIG. 1) into a single frame referred to as the block ACK, and transmits the frame to the transmission station. A plurality of ACKs can thereby be transmitted in a single block ACK frame, and it is possible to reduce the header or the waiting time for media access.
In the delayed block ACK scheme as shown in FIG. 2, when obtaining transmission opportunity, the transmission station transmits a plurality of units of QoS data in sequence, and transmits a block ACK Request for requesting transmission of a block ACK to the receiving station as the final sequential transmission. Then, when receiving the block ACK request, the receiving station returns a normal ACK frame for the block ACK Request to the transmission station without immediately returning a block ACK. When the receiving station obtains transmission opportunity next time shortly thereafter, the receiving station transmits a block ACK frame. The transmission station returns an ACK to the receiving station when receiving this block ACK frame. The delayed block ACK scheme thus temporarily opens the media when the priority of data transmission is not particularly high, or in such conditions as when the media is extremely congested. The delayed block ACK scheme is therefore flexible in comparison to the immediate block ACK scheme. Non-patent Document 1: “Draft Amendment to STANDARD Information Technology-Telecommunications and Information Exchange Between Systems-LAN/MAN Specific Requirements-Part 11: Radio Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Medium Access Control (MAC) Quality of Service (QoS) Enhancements,” IEEE P802.11e/D8.0, February 2004.